The invention relates to a method of manufacturing a colour display tube, in which a display window is provided with a display screen of phosphor elements luminescing in different colours, a tensed substantially rectangular shadow mask which is provided with a large number of apertures is suspended in the display window, and an enveloping part is secured to the display window, such that an envelope is formed.
The invention also relates to a colour display tube produced by such a method.
A method of the type mentioned in the opening paragraph is known from U.S. Pat. No. 4,547,696, in which the display window and the shadow mask, the latter being provided on a frame, are first placed in a defined position relative to one another by means of a first plurality of reference cavities in the frame, a second plurality of reference cavities in the window and a number of spherical reference elements which fit in the reference cavities. A third plurality of reference cavities is provided on the frame to place the frame together with the display window in a defined position relative to a lighthouse. Subsequently, the phosphor elements of the display screen luminescing in different colours are provided on the display window by means of a multi-step photographic process, in which the display window is repeatedly detached from the frame and replaced in the same relative position by means of the spherical reference elements and the reference cavities. After the display screen is formed, a conical enveloping part the edge of which is provided with a fourth plurality of reference cavities, is placed on the frame by means of spherical reference elements which fit in the reference cavities, and secured thereto by means of a glass frit.
To obtain a satisfactorily operating colour display tube the ultimate positioning of the parts relative to one another must be sufficiently exact. To this end, the reference means used for positioning the parts relative to one another must be manufactured accurately such that the parts can be reproducibly positioned relative to one another in the accurately defined position.
Not only in the method as described in U.S. Pat. No. 4,547,696 but also in other known methods of manufacturing a colour display tube, the tube is heated to approximately 400.degree. C. during securing the enveloping part to the display window and during evacuating the envelope of the colour display tube. During this heating step the shadow mask expands and it has been found in practice that when the colour display tube has cooled the shadow mask does not always resume its original position. Owing to this, the apertures in the shadow mask may be displaced relative to the phosphor elements of the display screen, thereby causing colour errors in the colour display tube.